Balloon bombs were launched by the Japanese army between November 1944 and July 1945 in Operation fu-go and carried by the jet stream across the Pacific Ocean, traveling from Japan to North America. The operation was planned by Colonel Kusaba Sueki of the 9th Research Division in reprisal for the Doolittle raid on Tokyo on 18 April 1942. The original plan was to launch the balloon bombs from submarines near the U.S. West Coast, but the Japanese navy refused to provide submarines for such a mission. As a consequence, plans were developed to launch the balloons from Japan.

The balloons were made of paper and konjak (a Japanese food made from taro paste, used as glue). They were about 30 ft in diameter and were filled with hydrogen. Each balloon carried one 33 lb antipersonnel bomb or two 13.2 lb incendiary bombs.

The balloons flew at a maximum speed of some 120 mph at an altitude of 24,000 to 30,000 ft in a flight that took about 50 hours. During the war, Japan produced 10,000 balloon bombs and launched 9,000 of them against North America, but only about 285 incidents involving them were reported in the United States and Canada. The balloons had no sensor or guidance systems, and once they were released, their flight depended entirely on the jet stream air currents. Therefore, the bombs rarely hit their targets.

In the only reported fatalities from the balloon bombs, one woman and five children were killed while on a picnic in Oregon on 5 May 1945. Some West Coast forest fires were attributed to the balloon bombs, but there is no direct evidence of this. The bombs reached British Columbia in Canada, and Alaska, Oregon, Montana, Washington, California, north Texas, and even Michigan in the United States.

The U.S. government feared that Japan would use the balloons as biological weapons, and it prohibited press reports about them to prevent panic among its citizens. Indeed, the Japanese made preparations for just such an effort, although it was never carried out. The U.S. military, meanwhile, endeavored to locate the balloon launch sites, which were finally identified by analyzing a balloon captured by the U.S. Navy. That the balloons had come across the Pacific from Japan was a considerable surprise.

The Japanese army expended enormous resources in Operation fu-go. Some 30,000 Japanese soldiers and almost 30,000 civilians were engaged in the operation over a two-year period, and six soldiers were killed in an accident while loading bombs for the balloons.